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This study investigated material flow behaviour in similar aluminium alloy and magnesium alloy sheet lap welds produced by friction stir spot welding. The material flows around the tool pin and shoulder were visualised through the tracer material technique, and schematic illustrations were made to clarify the induced material flow direction and intensity. The material flow observations described that the pin part provided the outward spinning motion of the material, while the shoulder part generated a centrifugal effect at the edge for downward flow/hooking. Induced material flow intensity suggested that dominant flow patterns near the shoulder edge were observed for the similar Al welds, whereas high intensity outward spinning flows at the pin periphery for the similar Mg welds. As a result, the material flow under the pin and shoulder of the tool explained that the morphology of the failure mode was highly dependent on the tool geometry and the alloy material adopted.
Leon et al. (Mon,) studied this question.